Ideally, a wind turbine blade of the airfoil type is shaped similar to the profile of an aeroplane wing, where the chord plane width of the blade as well as the first derivative thereof increase continuously with decreasing distance from the hub. This results in the blade ideally being comparatively wide in the vicinity of the hub. This again results in problems when having to mount the blade to the hub, and, moreover, this causes great loads during operation of the blade, such as storm loads, due to the large surface area of the blade.
Therefore, over the years, the construction of blades has developed towards a shape, where the blade consists of a root region closest to the hub, an airfoil region comprising a lift-generating profile furthest away from the hub and a transition region between the root region and the airfoil region. The airfoil region has an ideal or almost ideal blade shape with respect to generating lift, whereas the root region has a substantially circular cross-section, which reduces the storm loads and makes it easier and safer to mount the blade to the hub. The root region diameter is preferably constant along the entire root region. Due to the circular cross-section, the root region does not contribute to the energy production of the wind turbine and, in fact, lowers this a little because of drag. As it is suggested by the name, the transition region has a shape gradually changing from the circular shape of the root region to the airfoil profile of the airfoil region. Typically, the width of the blade in the transition region increases substantially linearly with increasing distance from the hub.
As for instance blades for wind turbines have become bigger and bigger in the course of time, and they may now be more than 60 meters long, the demand for optimised aerodynamic performance has increased. The wind turbine blades are designed to have an operational lifetime of at least 20 years. Therefore, even small changes to the overall performance of the blade may over the lifetime of a wind turbine blade accumulate to a high increase in financial gains, which surpass the additional manufacturing costs relating to such changes. The focus areas for research have been directed towards improving the airfoil region of the blade for many years, but during the recent few years, more and more focus has been directed towards also improving the aerodynamic performance of the root and transition regions of the blade.
WO2007/065434 discloses a blade wherein the root region is provided with indentations and/or projections in order to decrease the drag from this part of the blade.
WO2007/045244 discloses a blade, wherein the root region and the transition region is designed so as to have at least two separate airfoil profiles in order to increase the lift of these regions.
WO0208600 describes a wind turbine, where the output of the wind turbine is increased by providing the root section of a wind turbine with a member that is designed in such a way that the assembly consisting of the member and the root section can absorb wind energy and increases the overall efficiency of the wind turbine.
WO2007/118581 discloses a blade, where the inboard part of the blade is provided with a flow guiding device on the pressure side of the blade in order to increasing the aerodynamic performance of the blade by increasing the lift. However, the design proposed is very rigid due to the triangular shaped cross-section and consequently the flow guiding device has a tendency to separate from the surface of the blade, when the blade bends.
EP 1 845 258 discloses a blade having a Gurney flap like device arranged in the transition portion of the blade. The Gurney flap like device has a concave curvature and is arranged at the trailing edge on the pressure side of the blade.